For many years it has been desired to produce membranes that can separate chemical species of similar size efficiently. Typical applications include gas separations (such as O2/N2 and CH4/CO2), the removal of organics and salts from water by reverse osmosis, and the separation of ethanol and water by pervaporation.
Polymeric membranes made by conventional processes such as interfacial polymerization, phase inversion, and evaporative casting have long been used for separation applications; however, researchers and manufacturers have not been able to obtain sufficiently narrow pore-size distributions to achieve the efficient separation of chemical species of almost identical size. This is partially due to the pore size distribution found in polymeric membranes as a result of polymer chain packing. Because polymers are not 100% crystalline, entropic effects cause some interchain voids to be larger than others. Furthermore, the general inability of a polymer membrane to restrict rotational degrees of freedom of larger molecules while allowing unrestricted movement of smaller molecules makes it difficult to prevent the diffusion of larger molecules across the membrane. These phenomena result in a flux/selectivity tradeoff that limits the effectiveness of polymeric membranes.
Attempts have been made to achieve more refined size-based separations by using structured inorganic materials, such as zeolites, in the form of flat sheet membranes. These membranes show great potential due to their narrow pore size distributions. However, due to their fragile nature and difficult formation procedures, they have not yet been widely used. Furthermore, since these membranes cannot be made in hollow fiber form (which results in a high surface area to volume ratio), their surface area to volume ratio also limits their use. In applications that require superior chemical and thermal stability, ceramic membranes have proven to be better than polymer membranes. However, these membranes are often expensive, difficult to produce, and fragile.
Over the past decade, mixed matrix membranes have been proposed as an answer to the above membrane drawbacks. By suspending zeolite particles in a continuous matrix of low permeability polymer matrix, it has been possible to achieve separations not possible by polymer membranes. The improvement is gained from suspended zeolite particles locked into the polymer matrix. These particles are chosen to substantially decrease the permeability of one chemical component while increasing the permeability of another. Thus, the desired permeate component moves faster through a mixed matrix membrane than through a purely polymeric membrane made with the same polymer. Additionally, the undesired chemical component is forced to travel a more tortuous path around the zeolite particles, thus decreasing mobility for that component and increasing the overall selectivity for the desired component. Like zeolite films, however, mixed matrix membranes are not without drawbacks. For example, such mixed matrix membranes are limited in their separation capabilities. Because the zeolite particles are by no means a continuous separation layer, only a small improvement over polymeric membranes may be achieved. Furthermore, many researchers have dealt with poor polymer-zeolite adhesion, which results in decreased selectivity.
Numerous membranes have been used to varying degrees of success for separations; however, more refined size separations remain the goal of considerable ongoing research. Therefore, the remaining challenge is to produce a membrane with the effectiveness of a continuous zeolite sheet, but with the flexibility and durability of a mixed matrix membrane.